IMPULSE DRYER ROLL WITH SHELL OF HIGH THERMAL DIFFUSIVITY

Abstract: In a paper-making machine a roll (22) is composed of two parts: a metallic base shell (38), which is constructed of conventional spun-cast steel alloy; and a thin outer shell (40) constructed of a material of high thermal diffusivity. The outer shell (40) is one to a few tenths of an inch thick and is in intimate contact with the surface of the steel alloy base shell (38). Typical materials of high thermal diffusivity for uses in the outer shell (40) are copper and aluminum. The roll (22) may be formed by flame spraying a layer of copper approximately two-tenths of an inch thick on the surface of a steel alloy base shell (38). The roll (22) is employed where improved heat transfer to a paper web is desired for example in an impulse paper dryer, or in a calendar.

Description: PATENT APPLICATION IMPULSE DRYER ROLL WITH SHELL OF HIGH THERMAL DIFFUSIVITY FIELD OF THE INVENTION This invention relates to paper making machines and calenders. More particularly, this invention relates to rolls used in paper making machines which transfer heat to a paper web. Still more particularly, this invention relates to a heated roll used in a paper making dryer or calender which has improved heat transfer to the paper web. BACKGROUND OF THE INVENTION Paper manufacture is a capital intensive industry. A drive for increased productivity has led to efforts to increase paper production by increasing the width of the paper web which can be made on a paper making machine. Another trend for increased productivity is to increase the speed of the web through the machine. The width of the web can be increased by making the machine and the paper handling rolls wider. Increasing the speed at which the paper web is manufactured creates a problem in the dryer section of the paper making machine. As web speed increases, heat transfer to the dry paper web from each dryer cylinder decreases. Thus, to deal with higher web speed, dryer sections of paper making machines have had to be made longer. One solution to the problem of longer dryer sections is the impulse dryer. The impulse dryer employs a high temperature roll which is heated to 450° F or higher. The heated roll is used to form an extended nip between the roll and a shoe. The paper web is brought into contact with the high temperature surface of the roll. The paper is backed by a press felt which in turn overlies a blanket which rides a lubricating film over the shoe. By using such extended nip impulse dryers it has been possible to significantly decrease the length of the dryer section of a paper making machine. Further improvements in the impulse dryer require an increased heat transfer between the roll and the paper web. Increasing the roll surface temperature necessitates increasing the temperature of the entire roll. This has deleterious effects on the roll strength. Increased roll temperature can also complicate the use of internal crown support mechanisms which rely on hydraulics. Further, high temperatures can cause searing of the press felt, picking of the web surface fibers, excess roll head stresses, and overheated bearings. In U.S. patent No. 4,324,631 to Wahren, it is recognized that an impulse dryer roll would benefit from being made of a material of relatively high thermal conductivity. Wahren also teaches the desirability of a roll of low surface conductivity to achieve high surface temperature. Wahren proposes a material selection criteria for at least the surface layer of the roll which does not favor aluminum over steel or nickel. Wahren discounts copper as not suitable for his roll. U.S. Patent No. 4,631 ,794 to Riihinen, describes a roll for use in a calender which is inductively heated. The roll uses a non-magnetic insulating layer between an outer, ferromagnetic layer and the inner core of the roll. Riihinen provides inductive heating of the roll in a calender and allows access to the interior of the roll for crown control, but does not disclose how to use increased thermal conductivity to improve heat transfer. U.S. Patent No. 4,738,752 to Busker et al suggests a press roll for use in a heated, extended nip press which includes a first co-axial layer, and a second co-axial layer extending about the first layer. The second layer has a coefficient of thermal conductivity greater than the coefficient of thermal conductivity in the first layer. The first layer is a material having a low coefficient of thermal conductivity and the second layer is metallic. It is further suggested that the first layer may be ceramic and the second layer metallic. Busker et al suggests that the second outer layer has a thickness in the range of 0.005 inches to 0.050 inches. Busker et al does not disclose how to calculate the proper thickness of a metal outer shell over an inner metal base shell. Nor does Busker teach the importance of thermal diffusivity in the choice of material for and the thickness of the outer shell. What is needed is a roll for use in an impulse dryer or calender with increased heat transfer capability. SUMMARY OF THE INVENTION The roll of this invention is composed of two parts. One is a metallic base shell, which is constructed of conventional, spun-cast steel alloy. The second part of the roll consists of a thin outer shell a few tenths of an inch thick which is in intimate contact with the surface of the steel alloy base shell. The outer shell is constructed of a material of high thermal diffusivity. Thermal diffusivity is proportional to the thermal conductivity, and inversely proportional to the specific heat and density of a material. Examples of materials of high thermal diffusivity are copper and aluminum. One way to construct the roll of this invention is to flame spray a layer of copper approximately two tenths of an inch thick on the surface of a roll. The roll is employed in the extended hot press nip of an impulse dryer. The extended nip is formed by a shoe. The shoe has a smooth, arcuate surface of slightly greater curvature than the surface of the roll of this invention. An extended nip blanket is drawn over the surface of the shoe at a speed equal to the surface speed of the roll. A paper web travels adjacent to the roll surface, and a press felt is positioned between the blanket and the web. The effect of the shoe is to hold the paper web against the roll over a circumferential distance of perhaps ten inches on a five foot diameter roll. The roll is rotated to produce a surface velocity on the order of 3,000 feet per minute. The extended nip blanket, press felt and web of paper all move at an identical speed past the roll. The roll is constructed of a heavy-walled metal cylinder. The cylinder wall is typically several inches in thickness. The roll is heated by induction heaters which heat the surface of the roll upstream of the extended nip. When an analysis is performed on the heat transfer between the roll and the web, it is observed that only the outermost portion of the roll experiences a fluctuating temperature. If the surface of the roll is heated to 450° Fahrenheit by induction heaters, the entire interior of the roll will reach an equilibrium temperature of 450° F. The surface of the roll that comes into contact with the web may experience a temperature drop at the surface of 200° F or 300° F. However, this temperature fluctuation is only experienced by the top one- to three- tenths of an inch of the roll. Thus, the effective heat transfer portion of the roll is only a thin, outer annulus. In a conventional roll, the only way to increase heat transfer is to increase roll temperature, increase nip length, or reduce web speed. Because all three of these approaches have costs and disadvantages associated with them, the present invention increases the heat transfer between a roll and the web by increasing the thermal diffusivity of the role by constructing it to include a copper layer of a precisely calculated depth. It is an object of the present invention to provide an impulse dryer roll with increased heat transfer to a paper web being dried. It is another object of the present invention to provide a roll for use in a calender which provides increased heat transfer to a paper web for a given roll surface temperature. It is a further object of the present invention to provide a roll for use in the dryer section of a paper making machine which requires a shorter section. It is a still further object of the present invention to provide a roller for use in an impulse dryer which reduces picking of the web surface fibers. It is a still further object of the present invention to provide a roller for use in an impulse dryer that reduces roll head stress. Further objects, features, and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional elevational view, somewhat in schematic form, of a wide area, or long nip or extended nip type, paper making machine which employs the two-layer roll of this invention. FIG. 2 is a cross-sectional elevational view, somewhat in schematic form, of a wide area, or long nip or extended nip type paper making machine which employs a stretched, extended nip blanket, and utilizes the roll of FIG. 1 . FIG. 3 is a graphical view of temperature versus time at various depths from the surface of the roll of FIG. 1 . FIG. 4 is a cross-sectional elevational view, somewhat in schematic form, of a calender which employs the two-layer roll of this invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring more particularly to FIGS. 1 -4, wherein like numbers refer to similar parts, an impulse dryer 20 is shown in FIG. 1 . The impulse dryer 20 includes a press roll 22 which forms a nip 26 with a shoe 24. The shoe 24 is provided with a concave surface facing the roll 22 and is mounted so that it is urged towards the roll 22. The press nip 26 is formed between the roll 22 and the shoe 24. A web of paper 28 passing through the nip 26 is subjected to a pressing pressure over an extended length of time. A press felt 32 moves beneath the web 28 and a looped belt 30 passes through the nip over the shoe 24 beneath the felt 32. Oil is supplied between the shoe 24 and the belt 30. The oil causes a hydrodynamic wedge of fluid to build up between the belt 30 and the shoe 24. The fluid wedge transmits pressure to the web while at the same time lubricating the movement of the web 28 through the nip 26. The press felt 32 passes through the nip 26 while underlying the paper web 28 and riding on the belt 30. The paper web 28, the press felt 32, and the belt 30, as well as the roll 22, are in engagement and so driven at the same speed. Thus the paper web 28 does not experience significant sheer force because there is no relative motion in the plane of the web 28 and the press felt 32 and the surface 34 of the press roll 22. Thus the paper web 28 is subject to principally compressive forces as it moves through the extended nip 26. The effect of this compressive force is to bring the web into intimate contact with the surface 34 of the press roll 22. The intimate engagement of the web 28 with the press roll surface 34 under pressure facilitates the rapid heat exchange between the surface 34 of the roll 22 and the web 28. The rapid heat transfer between the roll 22 and the web 28 produces a not completely understood drying mechanism which is characteristic of the impulse dryer. The rapid heating of the paper web vaporizes some of the water contained in the web. The steam which has been produced from the water in the web is trapped between the surface 34 of the roll 22 and the paper web 28. Its only route of escape is through the paper web 28 into the press felt 32. The rapid downward movement of the steam from the upper surface of the paper web 28 downwardly into the press felt 32 has the effect of blowing water contained in the web 28 into the felt 32. This process, impulse drying, results in the rapid removal of water from the paper web 28. The press roll 22 in the impulse dryer 20 has improved effectiveness over a conventional impulse dryer because of the construction of the roll 22. The roll 22 has a metallic base shell 38 constructed of conventional steel alloy. The base shell 38 is overlain by an outer shell 40 which is constructed of a material of high thermal diffusivity such as copper. It should be noted that the rolls as illustrated in the Figures are not precisely to scale, and for illustrative purposes that relative thicknesses of the base shell and the outer shell may be exaggerated. The outer shell 40 is intimately joined to the outer surface 42 of the base shell 38. The metal base shell 38 forms the structural support for the outer shell 40. The increased thermal diffusivity of an outer copper layer can double the effective heat transfer between the roll 22 and the paper web 28. A typical impulse dryer consists of a press nip in which the press roll reaches a surface temperature in excess of 200° to 300° F and preferably between 450° and 550° F. A typical impulse dryer may employ an extended nip press employing a shoe which presses an impermeable blanket against an inductively heated roll. The high surface temperature rapidly heats the wet web as it passes through the nip and softens the paper fibers. This greatly enhances the removal of water and the development of sheet strength properties. The ability to increase the heat transfer by using a thin outer shell 40 of increased thermal diffusivity allows impulse dryers to be designed to achieve selected advantageous effects. First, the surface temperature of the roll 22 may be lowered and still provide the same level of heating. This overcomes several problems associated with high shell temperatures. High temperatures can cause searing of the press felt, picking of the web surface fibers, excess roll head stresses and overheated bearings. Second, more heat can be supplied to the paper to remove more moisture. Third, the speed of the web 28 may be increased, while holding input and drying effectiveness constant. In actual optimized designs employing the high thermal diffusivity outer shell press roll 22, all three advantages of lower press roll temperature, higher operating speed, and more complete drying of the paper web 28 may be incorporated. FIG. 1 is a cross-sectional view of an impulse dryer 20 employing the improved press roll 22 and taken along the direction of travel of the paper web 28. As will be appreciated by those versed in the art of paper making, the cross-machine width of the paper web 28 will normally be between one-hundred and four-hundred inches, with the components of the impulse dryer such as the roll 22 being in general somewhat longer, as necessitated by their particular function. The looped belt 30 and its support 44 are conventional and are described more fully in U.S. Patent No. 4,673,461 (Roerig et al). The belt 30 is a continuous loop. It has a cross-machine width greater than the press roll 22, so that the ends of the belt (not shown) may be sealed to circular closures (not shown) which seal the ends of the belt, thus containing the nip lubricating oil within the sealed belt 30. A stationary beam 33 is contained within the belt 30. The beam adjustably supports the shoe 24 by means of a hydraulic piston chamber 35 in which is positioned a piston 37. The shoe is pivotally supported on a roller pin 39, seated in a downward facing groove in the shoe 24 and an upward facing groove in the piston 37. The piston is urged upward by fluid pressure beneath the piston in the chamber 35, which is in the form of an elongated slot, slidably receiving the piston, and extending the full width of the machine beneath the shoe 24. The belt 30 is guided by means of curved side guides 41 and 43, upper guides 45 and 47, and a lower guide 49. The guides are adjustably mounted to the beam 33 and serve to stabilize the belt 30 during start-up. The guides also stabilize the belt 30 if any fluttering or instability should occur during normal operation. Once the belt 30 has reached operational speed, centrifugal force will cause the belt 30 to assume a naturally circular shape, except where traversing the nip 26 between the shoe 24 and the press roll 22. Thus, at speed, the centrifugal forces will normally hold the belt 30 a short distance off the guides, 41 , 43, 45, 47, and 49. The press felt 32 is supported on an infeed roll 46 and a press outfeed roll 48. The infeed press felt roll 46 and outfeed roll 48 will typically have a diameter of two feet, where the corresponding diameter of the press roll is five feet. The rolls 46, 48, serve to bring the press felt into position to be fed through the nip 26 of the impulse dryer 20. The press felt 32, after leaving the outfeed roller 48, is processed by a felt dryer (not shown), which removes water and excess moisture from the felt 32 before it returns for reuse over the infeed roller 46. The press roll 22 in FIG. 1 is shown employing a hydraulic crown control mechanism 50 which has a non-rotating crown support beam 52. The crown support beam has an oil supply port 54 which supplies oil to piston cavities 56 which drive pistons 58 against the inner surface 60 of the metallic base shell 38. Pistons 58 which are spaced along the central beam or shaft 50 serve to apply a constant pressure between the press roll 22 and the shoe 24. In FIG. 1 , an induction heater 62 is shown schematically. It has coils 64 which are energized with high frequency current. The induction heater 62 is conventional in nature. It employs oscillating magnetic fields caused by the high frequency alternating current, which create eddy currents in the surface 40 of the roll 22. The currents induced produce resistance heating in the surface 40 thereby heating it to the desired temperature. From a heat transfer point of view, the shell 40 appears to be a "semi-infinite plate." This is because of the very short time a given segment of the roll is in the nip. With external induction heating of the shell 40, the entire roll operates with a uniform temperature from the inside 60 to the outside 34, with the exception of a thin outer layer which undergoes cyclical temperature fluctuations. The effective depth of this outer layer can be estimated according to the following expression: x= Where: X = the depth of the layer which experiences cyclical temperature variations in ft. S = the press roll surface speed in ft/hr. L = the nip length in ft. a = thermal diffusivity of the outer surface in ft 2 /hr. This depth is estimated based on the assumption that the surface experiences a step-change in temperature. In practice, the change will not be this dramatic, so the above estimate will tend to be on the high side. FIG. 3 is a graphical representation of the temperature fluctuations with time of the surface of the roll and three points at successively greater depths. In FIG. 3, the vertical axis is temperature and the horizontal axis is time. The uppermost curve 66 represents the temperature on the surface of the roll versus time. Because the press roll 22 is approximately five feet in diameter, it has a circumference of approximately 1 6 feet. The nip, 26, shown in FIG. 1 , is ten inches long, with the result that the roll is exposed to the paper web 28 for approximately five percent of the time as it revolves around. The upper curve 66 has a repeated saw-tooth wave form 68 which represents the surface temperature over an entire rotation of the drum 22. Taking as the starting point the precipitous drop in temperature 70, which represents the contact of the surface 34 of the roll 22 with the paper web 28, on the scale of FIG. 3 where the entire web contact time represents five percent of a wave-length 68, the drop in temperature of the surface is essentially instantaneous. The surface 34 of the roll 22 then maintains the temperature of the paper of the web 28 as it is heated by contact with the roll. As a point on the surface exits the nip, it experiences a gradual increase in temperature 72 as heat flows from the interior of the roll towards the lower temperature surface. Finally, the roll surface experiences at 74 a steep increase in temperature as it moves past the " ■ ~ " " pc induction heater 62. Thereafter, the temperature remains constant until it again at 70 comes into contact with the paper web 28. The second graph 75 represents the temperature profile of an arbitrary point some distance removed in depth from the surface 34 of the roll 22. The wave length 76 of the second curve 75 is the same as the wave length 70 of the surface temperature curve 66. However, the change in temperature is less abrupt as the point represented by the curve 75 is somewhat removed from the surface. A third curve 78 has a wave length 80 which is identical to the wave lengths 76, 68, and is governed by the rotation rate of the roll 22. This curve represents a point further removed from the roll than the curve 75 and shows how, at greater depths within the roll, the changes in temperature caused by the cyclical nature of the surface 34 coming in contact with the roll and being heated by the induction heater 62 are damped out. The fourth curve 82 shows a constant temperature with time. It represents the approximate depth at which the above equation predicts no fluctuation in temperature. It is also the depth at which the thermal diffusivity of the material is no longer important in governing heat flow between the roll 22 and the paper web 28. Thermal diffusivity (a) is defined as: Where: a = thermal diffusivity. k = thermal conductivity. c = specific heat. p = density. An alloy cast iron shell has a thermal diffusivity of only about 0.6 to 0.7 ft 2 /hr, whereas the thermal diffusivity of aluminum is 3.3 and copper is approximately 4.4. TABLE 1 Thermal Diffusivity Nip Length Web Speed Effective Depth ( ft 2 /hr.) (Inches) (Ft./min.) (Inches) 0.7 10 3,000 0.086 2 10 3,000 0.146 3 10 3,000 0.179 4 10 3,000 0.206 0.7 1 3,000 0.027 2 1 3,000 0.046 3 1 3,000 0.060 4 1 3,000 0.065 0.7 10 6,000 0.061 2 10 6,000 0.103 3 10 6,000 0.126 4 10 6,000 0.146 Referring to Table 1 , column 1 has various values of diffusivity; 0.7 corresponding roughly to the typical material from which press rolls are constructed. A thermal diffusivity of 3, corresponding to aluminum, and a thermal diffusivity of 4, corresponding to copper. In Table 1 , two common nip lengths in inches are listed, with 10 corresponding to a typical impulse dryer extended nip, and one inch corresponding to a typical calender application as shown in FIG. 4. Two web speeds in feet per minute are listed. Three thousand feet per minute is a typical speed for modern paper making machines. Six thousand feet per minute is the approximate upper range of paper speeds currently being discussed by the industry. Table 1 indicates that the approximate maximum radial thickness of the outer surface shell 40 need be only approximately two-tenths of an inch thick with a combination of thermal diffusivity, nip length, and web speed as shown. Taken in conjunction with FIG. 3, Table 1 also illustrates the two layer construction of a press roll 22. The volumetric specific heat, i.e., the amount of thermal energy per unit volume, is very roughly constant over a broad range of metals, inasmuch as low density metals tend to have high specific heat and the high density metals have a low specific heat but more material per unit volume. Thus, the volume of material experiencing cyclical heating is a good indicator of the total amount of heat being supplied by the roll 22 to the paper web during its cyclical contact with the paper web 22. FIG. 3 may be viewed with the understanding that the depth of the points corresponding to curves 75, 78 and 82 double as the effective depth, as shown in Table 1 , doubles. Thus, the movement from an alloy steel roll with an effective depth of eighty-six-thousandths of an inch, to copper with an effective depth of 207-thousandths of an inch means that over twice the volume of material is undergoing cyclical temperature fluctuation. Thus, the heat flow during a given cycle is roughly twice as high. Another observation which can be gleaned from Table 1 is that as the nip length decreases the effective depth decreases, so that in a calender, as shown in FIG. 4, where the nip 76 is in the range of one-half to one-and-a- half inches, very thin, high conductive outer shells 78 are effective at dramatically increasing heat transfer. Press rolls 22 are conventionally manufactured by centrifugal casting of iron alloy against a chilled mold. Centrifugal casting against a cold metal shell typically results in a roll with high surface hardness and internal ductility. The outer shell of higher thermal diffusivity material such as copper or aluminum will preferably be sprayed onto the exterior surface 42 by spraying molten metal on the surface. Typical metal spray techniques involve sand blasting the surface of the roll to develop a surface to which the sprayed metal will adhere, heating a wire of the desired metal through an electrical arc or plasma, and spraying the resulting metal on the surface 42 of the metal base shell 38. Typically, a metal spraying operation would be performed while the base shell 38 is rotating, with successive layers of sprayed metal applied until the requisite thickness is achieved. After this, the roll is surface finished to provide the surface smoothness required of a press roll in an impulse dryer or a calender roll in a calender. One possible method of forming the exterior shell involves centrifugally casting a thin outer layer first, perhaps by spraying molten metal into the chilled centrifugal mold, then casting the steel base shell 42 onto the outer shell 40. This method would require preventing an oxidized coating from forming on the outer shell 40, so as to produce a molecular bond between the outer shell 40 and the base shell 34. Other possible methods include chemical or electrical deposition from solution onto the surface 42 of the base shell 38. In chemical deposition, it is important to avoid porosity in the deposited metal. Another possible method, especially for very thin layers such as used in a calender, would be vapor deposition. FIG. 2 shows an alternative impulse dryer 120 which has a press roll 122, and forms and extended nip 126 between a shoe 124 and the surface 134 of the roll 122. The impulse dryer 120 employs an impervious extended nip blanket 130. The shoe 124 is provided with a concave surface facing the roll and is mounted so that it is urged towards the roll 122. The nip 126 subjects a web of paper 128 passing through the nip to a pressing pressure over an extended length of time. The nip blanket 1 30 passes through the nip between the shoe 124 and the roll 122. Oil is supplied between the shoe and the blanket 130. The oil causes a hydrodynamic wedge of fluid to build up between the blanket 130 and the shoe 124. The fluid wedge transmits pressure to the web while at the same time lubricating the blanket's movement through the nip 126. A press felt 132 passes through the nip 126 while underlying the paper web 128 and riding on the blanket 130. The paper web 128, the press felt 132, and the blanket 130, as well as the roll 122, are driven at the same speed by their inter-engagement through frictional force. Thus, the paper web 128 does not experience significant sheer action because there is no relative motion in the plane of the web 128 and the press felt 132 and the surface 134 of the press roll 122. Thus, the paper web 128, is subject to principally compressive force as it moves through the extended nip 126. The effect of this compressive force is to bring the web into intimate contact with the surface 134 of the press roll 22. In a way similar to the dryer 20, the press roll 122 in the impulse dryer 120 has improved effectiveness over a conventional impulse dryer. The roll 122 has a metallic base shell 138 constructed of conventional steel alloy. The base shell 138 is overlain by an outer shell 140 which is constructed of a material of high thermal diffusivity. The outer shell 140 is intimately joined to the outer surface 142 of the base shell 38. The metal base shell 138 forms the structural support for the outer shell 140. The blanket 130 and its support 44 are conventional. The blanket 130 is a continuous loop. It has a cross-machine width greater than the press roll 122, so that the edges of the belt (not shown) extend past the edges of the paper web 128. A stationary beam 133 is beneath the belt 130 and the shoe 124. The beam adjustably supports the shoe 124 by means of a hydraulic piston chamber 135 in which is positioned a piston 137. The piston 137 is urged upward by fluid pressure beneath the piston 137 in the chamber 135, which is in the form of an elongated slot, slidably receiving the piston, extending the full width of the machine beneath the shoe 124. The belt 130 is guided by means of upper guide rolls 141 and 143, and lower guide rolls 145 and 147, and a stretcher roll 149. The stretcher roll 149 is adjustably mounted to vary tension in the blanket 130. The press felt 132 is supported on an infeed roll 146 and a press outfeed roll 148. The infeed press felt roll 146 and outfeed roll 148 will typically have a diameter of two feet, where the corresponding diameter of the press roll is five feet. The rolls 146, 148, serve to bring the press felt 132 into position to be fed through the nip 126 of the impulse dryer 120. The press felt 132, after leaving the outfeed roller 148, is processed by a felt dryer (not shown), which removes water and excess moisture from the felt 132 before it returns for reuse over the infeed roller 146. The press roll 122 in FIG. 2 is not shown employing a hydraulic crown control mechanism. Rather the roll 122 is formed with a diameter which is greater in the center and which tapers axially. The disadvantage of using a crowned roll 122 is that the pressure is only created for one level of pressure between the roll 122 and the shoe 124. Thus it may be desirable to employ an active crown if a range of drying pressures is desired. In FIG. 2, an induction heater 162 is shown schematically. It has coils 164 which are energized with high frequency current. The induction heater 162 is conventional in nature. It employs oscillating magnetic fields caused by the high frequency alternating current, which create eddy currents in the surface 141 of the roll 122. The currents induced produce resistance heating in the surface 134 to the desired temperature. FIG. 4 illustrates the use of a roll 86 which has an outer shell 87 of high thermal diffusivity in a calender 84. Calenders are used to smooth and improve the surface finish of a paper web 82. FIG. 4 is a schematic illustration of a machine stack, or "on machine" calender 84. The calender comprises a calender stack, constituted by calender rolls 86, 88, 90, and 92. Calenders may typically have a hard nip between hard rollers or may have a soft nip wherein the hard rollers 86, 90 are alternated with soft rollers 88, 92 composed of elastic fiber, the construction being conventional. A soft nip calender typically has nips 94 of one-half to one-and-a-half inches in length. The calender roll 86 may be heated by infrared or induction heating. When a calender roll having a metal base shell 96 and an outer shell 87 with high thermal diffusivity is used, improved heat transfer is effected between the roll 86 and the paper web 98. This increases the effectiveness of the calender 84. As shown in Table 1 , calenders, because of their shorter nip length, require thinner shells of high thermal diffusivity, in the neighborhood of 60-thousandths of an inch. The relatively thin radial thickness of the outer shell thus may facilitate the formation of the roll, being more adaptable to chem-film deposition. The relatively thin film also means that as non-metal, high thermal conductivity coatings such as diamond become available, it may be possible to coat rolls with materials such as vapor-deposited diamond. It should be understood that as the web speed increases, the required thickness of the outer higher thermal diffusivity shell decreases. It should also be understood that wherein the equation provided above for effective depth, as illustrated in Table 1 , gives maximum effective depth of heat transfer in the roll material, using depths less than the maximum effective depth will still serve to increase the heat transfer capability of a roll 22. It should also be understood that the nip length, roll diameters, and roll temperatures are illustrative, by way of example, and that other temperature ranges, roll size, and nip lengths could advantageously employ the roll 22 of this invention. It should also be understood that where a crown support mechanism is shown and illustrated, the roll could have a machined crown surface upon which is deposited or formed a thin outer shell of high thermal diffusivity. It should also be understood that wherein materials such as copper and aluminum have thermal diffusivities of three or more, thermal diffusivities in the range of one might be practical in some circumstances. It is understood that the invention is not confined to the particular construction and arrangement of parts herein illustrated and described, but embraces such modified forms thereof as come within the scope of the following claims.

Claims: CLAIMS I claim: 1 . A heated roll (22) for drying paper in a paper manufacturing machine comprising: a metallic base shell (38) composed of a first metal chosen for high strength and low thermal diffusivity, the base shell (38) defining a cylinder having a length and a radius and having a cylindrical surface, the base shell having a first thickness along the radial direction; an outer shell (40) of a second metal which overlies and is supported by the base shell (38), the second metal being chosen to have a higher thermal diffusivity than the first material, wherein the shell (40) has a radial thickness which is less than the radial thickness of the base shell (38), so that the base shell (38) provides support for mechanical loads and the outer shell (40) provides improved heat transfer therethrough. 2. The roll of Claim 1 wherein the radial thickness of the outer shell (40) is less than 0.3 inches. 3. The roll of Claim 1 wherein the radial thickness of the outer shell (40) is less than 0.1 inches. 4. The roll of Claim 1 wherein the first metal is a steel alloy and the second metal is selected from the group consisting of aluminum and copper. 5. The roll of Claim 1 wherein the outer shell (40) is formed of a metal spray of the second metal built up on the base shell. 6. A paper processing method comprised of the following steps: passing a paper web through a paper making machine in a first direction at a surface speed of S ft/hr; passing the paper web through a nip formed between a cooperating element and the surface of a roll, and wherein the nip has a length L measured in a first direction in feet; heating the surface of the roll so as to heat the paper as it transits the nip between the roll and the cooperating element, wherein the roll has an inner base shell of high strength and an outer shell of thermal diffusivity measured in square feet per hour of greater than one, and wherein the radial thickness of the high diffusivity layer is X measured in feet and wherein the expression determines the value of X. 7. A heated roll for treating paper in a paper manufacturing machine comprising: a base shell (38) composed of a first material chosen for high strength and having a low thermal diffusivity, the shell defining a cylinder having an internal radius and an external radius and having a cylindrical surface; an outer shell (40) of a second material overlying and supported on the base shell, the second material being chosen to have a higher thermal diffusivity than the first material, and the outer shell having an internal radius and an external radius, and wherein the difference between the internal and external radii of the outer shell is less than the difference between the internal and external radii of the base shell, such that the base shell provides support for mechanical loads, and the outer shell provides improved heat transfer to a paper web pressed into engagement therewith. 8. The roll of Claim 7 wherein the difference between the external radius and the internal radius of the outer shell is less than 0.3 inches. 9. The roll of Claim 7 wherein the first material is a steel alloy and the second material is selected from the group consisting of aluminum and copper. 10. The roll of Claim 7 wherein the outer shell is formed of a metal spray of the second metal built up on the base shell.

Inventors: WEDEL GREGORY L, WEDEL, GREGORY, L.
Applicants: BELOIT TECHNOLOGIES INC, BELOIT TECHNOLOGIES, INC.
Application year: 1994
Main classification: D21F3
IPC codes: D21F3, D21F5, D21G1